Problem: In math class, the girl to boy ratio is $10$ to $2$. If there are $20$ girls, how many boys are there?
Explanation: A ratio of $10$ girls to $2$ boys means that there is a group of $10$ girls for every group of $2$ boys. If there are $20$ girls, then there are $2$ groups of $10$ girls each. Because we know the ratio is $10:2$ , there must be $2$ groups of $2$ boys each. There is a total of $4$ boys in math class.